1. Field of the Invention
This invention relates generally to optical fiber communications and, more particularly, to a reduction in the effects of phase variations introduced by the optical carrier.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as SONET have been developed for the transmission of data over optical fibers. Typical communications systems based on optical fibers include a transmitter, an optical fiber, and a receiver.
The transmitter modulates the data, converts the data into an optical form and transmits the resulting optical signal across the optical fiber to the receiver. In a common design, the transmitter includes a laser source coupled to an external modulator. The laser source produces an optical carrier, which is modulated with the data by an external modulator. This results in an optical data signal which carries the data to be transported across the optical fiber.
On the other end of the optical fiber, the receiver recovers the original data from the optical data signal transported across the optical fiber. Recent advances in receiver technology are leading to more widespread adoption of receivers based on heterodyne detection. A heterodyne receiver typically includes a local laser source. The local laser source generates an optical local oscillator which is mixed with the incoming optical data signal as part of the heterodyne detection process.
To increase the efficiency of data transmission, many fiber communications systems utilize a coherent modulation scheme before transmitting the data on the optical data signal. A coherent modulation scheme takes advantage of phase information in a data signal.
Coherent modulation schemes are used in modulating data onto a transmitted optical signal and demodulating a received signal to obtain the transmitted data. For coherent modulation, an absolute phase reference is present at the transmitter and receiver to modulate and demodulated the transmitted signal, respectively. The phase references at the transmitter and receiver are said to be phase coherent when they are locked in phase.
A coherent modulation scheme utilizes phase information in representing data in a signal. Relative phases errors in the absolute phase references generally results in errors in the transmitted data. Example coherent modulation schemes include Quadrature Amplitude Modulation (QAM), Phase Shift Keying (PSK) and Quadrature Phase Shift Keying (QPSK).
An electrical signal generated by coherent modulation may be upshifted to an optical carrier to generate an optical signal employing coherent modulation. This optical signal can be generated using a laser source and an amplitude modulator (e.g. an MZM) to generate coherently modulated optical subcarriers. This approach, in which coherent modulation originates in the electrical domain, contrasts with direct coherent optical modulation, in which optical signals comprising coherent modulation can be generated using a phase modulator to modulate the optical carrier directly.
Data transmitted using coherently modulated optical signals can be received using heterodyne detection. Heterodyne detection is a type of coherent optical detection, or coherent detection, that generally utilizes an optical source as a local oscillator to downshift a coherently modulated optical signal to an RF signal. The local oscillator is generally at a different center frequency than an optical carrier or subcarrier. Subsequently, the RF signal is demodulated using a coherent local oscillator at an RF frequency. Data may also be coherently demodulated directly using homodyne detection, in which the optical local oscillator is phase coherent with the transmitting laser.
One particular optical communication process utilizes coherent modulation including optical subcarrier multiplexing at the transmitter and heterodyne detection at the receiver. Throughout this process, the data signal employing coherent modulation is converted from the electrical domain, to the optical domain, and then back to the electrical domain. The integrity of the data signal relies on the fidelity of the phase information in the data signal. At each step, phase noise can be introduced into the signal. Significant sources of phase noise in a transmission system include the phase noise of the transmit laser source producing an optical carrier and the phase noise in the receive laser source producing the optical local oscillator. Other sources of phase noise include the transmit local oscillator generating the electrical signal and the receive local oscillator demodulating the received electrical signal.
Traditionally, optical communication systems have addressed the problem of added phase noise by either using a phase-stable laser, or by designing complex phase-locking circuits into the demodulator. However, phase-stable lasers are both bulky and expensive, making them impractical for commercial applications. Complex phase-locking circuits are likewise expensive and difficult to implement. Furthermore, both solutions significantly raise the cost and complexity of the system.
Therefore there is a need for a fiber optic communications system which adequately deals with phase noise, including laser phase noise, without utilizing complex phase-locking circuits or expensive lasers.